(1) Field of the Invention
This invention relates to a process for the preparation of L type zeolites which belongs to the zeolite having .epsilon. cages, i.e., cancrinite cages.
(2) Description of the Prior Art
Zeolite is a crystalline aluminosilicate containing zeolitic water, as is seen from the fact that the origin of the word "zeolite" is a Greek word "Zein" meaning "boiling stone", and the composition of zeolite is generally expressed by the following empirical formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M stands for a metal cation having a valency of n, x is a number of at least 2, and y is a number of at least 0.
The basic structure of zeolite comprises SiO.sub.4 tetrahedrons having silicon at the center and four oxygen atoms at the apexes and AlO.sub.4 tetrahedrons having aluminum at the center instead of silicon, in which the tetrahedrons are regularly and three-dimensionally arranged connected to one another, while owning oxygen atoms jointly, so that the O/(Al+Si) atomic ratio is 2. Various three-dimensional network structures differing in the size and shape of fine pores are formed according to the manner of connection of the tetrahedrons. The negative charges of the AlO.sub.4 tetrahedrons are electrically neutralized by coupling with cations such as alkali metal or alkaline earth metal cations.
Fine pores thus formed have generally a size of 2-3 angstroms to ten-odd angstroms, and the size of the fine pores can be changed by exchanging the metal cations bonded to the AlO.sub.4 tetrahedrons with other metal cations having a different size.
Zeolite is used as a dehydrating agent, for a gas or liquid or a molecular sieve for adsorbing specific molecules while utilizing these fine pores. Furthermore, a product obtained by exchanging the metal cation with a hydrogen ion acts as a solid acid and is utilized as a catalyst utilizing the properties of the solid acid.
A typical composition of the L type zeolite is expressed as follows: EQU (K.sub.2,Na.sub.2)O.Al.sub.2 O.sub.3.6SiO.sub.2.5H.sub.2 O
and it is known that the SiO.sub.2 /Al.sub.2 O.sub.3 ratio varies between 5.2 and 7.0. The crystal structure of this zeolite comprises .epsilon. cages, i.e., cancrinite cages symmetrically piled with double 6-membered rings being interposed therebetween and is characterized in that it has primary fine pores of 12-membered rings of oxygen atoms having a diameter of 7.1 angstroms.
Various processes have been proposed as the process for the synthesis of the L type zeolite. These processes, however, are not completely satisfactory.
(1) According to the process disclosed in Japanese Examined Patent Publication No. 36-3,675, a potassium-containing reaction mixture obtained by using an aqueous colloidal silica sol as the silica source is crystallized at a temperature of 100.degree. to 200.degree. C. for such a long time as about 64 to about 169 hours. In this process, a long time is required for the synthesis and the yield of silica is very low.
(2) As an improvement of the process (1), Japanese Examined Patent Publication No. 46-35,604 discloses a process in which reactive amorphous solid silica is used as the silica source and the crystallization is carried out at a low alkali concentration. Indeed, the yield of silica is enhanced according to this process. However, expensive reactive amorphous solid silica is indispensably used as at least 80 mole % of the silica source and, if the amount of water contained in the reaction mixture is small, a long time is necessary for the synthesis.
It is known from the above known references and other literature references that in the synthesis of the L type zeolite having .epsilon. cages (cancrinite cages) in the crystal structure or an offretite type zeolite, a potassium ion is made present in the starting material mixture so as to facilitate the synthesis. The foregoing known processes were developed under this background, and in each of these known processes, it is intended to synthesize L type zeolites at a high efficiency by using an aqueous colloidal silica sol or reactive amorphous solid silica as the silica source.